31. Obovaria leibi:: (Lea) 



32. Eurynia nasuta (Say) 



33. Lampsilis siliquoidea rosacea (DeKay) 

 3h. Lampsilis ventricosa canadensis (Lea) 



35. Truncilla truncilla Rafinesque 



36. Truncilla donaciformis (Lea) 



37. Sphaerimn solidulum Prime 



38. Musculium transversum (Say) 



39. Pisidium compressum Prime 

 ho, Pisidivun scutellatum Sterki 

 Ul. Pisidium concinnulum Sterki 



Organisms taken in bottom sled 



Hauls of the bottom sled or Helgoland trawl in the ^Island 

 Section revealed the presence of some organisms which never appeared 

 in the quantitative dredges. Some of them vjere active swimmers which 

 live on or just above the bottom, while others were rare forms which 

 might be missed by samples covering only a small area. The form which 

 appeared most frequently in large numbers was the insect, Corixa, 

 Other insects which appeared only in the bottom sled were larvae of 

 the stonefly, Perla; and larvae of the mayflies, Heptagenia, Ephemera, 

 Ephoron, and Baetisca. Beetle larvae and dipterous pupae of unknown ^ 

 affinities were also taken. The crustacean, Mysis oculata relicta Loven, 

 V7as taken on several occasions. This form was also present in the 

 plankton. The following lamellibranchs appeared only in the bottom sled 

 collections: Obliquaria reflexa (Rafinesque), Ligumia recta (Lamarck), 

 and Amblema costata plicata (Say), In addition there were occasional 

 specimens of a mite (Eylais), an annelid (Sparganophilus) , and a 

 crustacean (Leptodora) . All organisms taken in the quantitative dredges 

 were taken also in the bottom sled, "~ 



Many of the hauls were made on firm bottom offshore. There was 

 no evidence that areas which had formerly had a bottom of sand, gravel, 

 or bouJ.ders had been covered by deposits of silt or organic debris. It 

 will be sho'.im later, also, that there was no evidence of pollution in 

 the relatively deeper areas with a mud bottom, 



258 



